1. Field of the Invention
The present invention relates to an improved tungsten electrode for a metal halide lamp and to an improved method of making the same.
2. Description of the Prior Art
Tungsten electrodes for use in metal halide lamps are well known in the art. Such tungsten electrodes are typically used in metal halide high intensity discharge lamps. In the production of tungsten electrodes, it is known to subject the electrodes to a heat treatment in the presence of wet hydrogen. It is also known to subject tungsten electrodes to a vacuum thermal treatment. It is known to treat tungsten electrodes (1) for 30 minutes at 1500.degree. C., (2) for 10 minutes at 2000.degree. C., and (3) at temperatures as high as 2800.degree. C.
In one known industrial process, tungsten electrodes are produced by first mixing a small amount of thoria powder, such as, for example, about 1% thoria powder, with tungsten powder and then cold pressing the mixture into a rod-like billet which is sintered at about 2100.degree. C. in hydrogen. After further processing to obtain individual electrodes, such electrodes are subjected to a wet hydrogen fire for about 5 to 10 minutes at about 1050.degree. C.
The foregoing process is known in the art. Such process produces acceptable tungsten electrodes for high intensity discharge metal halide lamps. However, it is an object of the present invention to provide an improved tungsten electrode. It is also an object of the present invention to provide a tungsten electrode which is more uniform than that produced heretofore. It is another object of the present invention to provide a tungsten electrode for use in a metal halide lamp having improved light output or luminous efficacy. It is a further object of the present invention to provide a tungsten electrode for use in a metal halide lamp having improved lumen maintenance during the life of the lamp.